Death can refer to an event or a state. Death as a state is the opposite of life; death as an event is the opposite of birth. The event of death is the termination of life in a living system, or in part thereof, while the state is that which follows.

Biologically, death can occur to wholes, to parts of wholes, or to both. For example, it is possible for individual cellss and even organss to die, and yet for the organism as a whole to continue to live; many individual cells can live for only a short time, and so most of an organism's cells are continually dying and being replaced by new ones.

Conversely it is also possible for the organism to die and for cells and organs to live and to be used for transplantation. In the latter case, though, the still-living tissues must be removed and transplanted quickly or they too will soon die without the support of their host.

Irreversibility is often cited as a key feature of death and, indeed, scientists have not been able to watch a living organism die and later bring it back to life. Nonetheless, many people do not seem convinced that death is always and necessarily irreversible; thus some have a literal belief in the resurrection of Jesus Christ, while others have high hopes for the eventual prospects of Cryonics.

The biological function of death is primarily to permit the operation of evolution.

Historically, attempts to define the exact moment of death have been problematic. Death was once defined as the cessation of heartbeat and breathing, for example, but the development of CPR and early defibrillation posed a challenge: either the definition of death was incorrect, or techniques had been discovered that really allowed one to reverse death (because, in some cases, breathing and heartbeat can be restarted). Generally, the first option was chosen. (Today this definition of death is known as "clinical death".)

Today, where a definition of the moment of death is required, we usually turn to "brain death" or "biological death": people are considered dead when the electrical activity in their brain ceases. It is presumed that a stoppage of electrical activity indicates the end of consciousness. Those that view that only the neo-cortex of the brain is necessary for consciousness, however, sometimes argue that only electrical activity there should be considered when defining death. In most places the more conservative definition of death (cessation of electrical activity in the whole brain, as opposed to just in the neo-cortex) has been adopted (for example the Uniform Definition of Death Act in the United States).

Even in these cases, the determination of death can be difficult. EEGs can detect spurious electrical impulses when none exists, while there have been cases in which electrical activity in a living brain has been too low for EEGs to detect. Because of this, hospitals often have elaborate protocols for determining death involving EEGs at widely separated intervals.

It might also be worthwhile to entertain the possibility that death does not occur at a particular moment, but unfolds as a process over a period of time. Perhaps, in the end, it is not terribly meaningful to speak of "the exact moment of death".

Death is also a popular mythological figure who has existed in mythology and popular culture since the earliest days of storytelling. The traditional image of Death is also a tarot card. See also: Death (personification)